The present disclosure relates generally to systems and methods for correcting dip measurements, and more particularly relates to systems and methods for interpreting for relative dip angles of formations or deviated wellbores utilizing vertical resistivity evaluation techniques.
The production of hydrocarbons from subsurface formations typically commences by forming a borehole into the earth to a subsurface reservoir thought to contain hydrocarbons. Tools may be deployed in the borehole to measure various physical, chemical, and mechanical properties of the formation, including for example, the porosity, permeability, saturation, and depth, of the subsurface formations encountered. This measurement includes induction logging to measure the conductivity or its inverse, the resistivity, of a formation by employing alternating currents to set up an alternating magnetic field in the surrounding conductive formation. This changing magnetic field induces detectable current loops in the formation.
Generally, a transmitter transmits an electromagnetic signal that passes through formation materials around the borehole and induces a signal in one or more receivers. The properties of the signal received, such as its amplitude and/or phase, are influenced by the formation resistivity, enabling resistivity measurements to be made. The measured signal characteristics and/or formation properties calculated therefrom may be recorded as a function of the tool's depth or position in the borehole, yielding a formation log that can be used to analyze the formation.
In vertical boreholes with little or no relative dip angle, a signal and response may be concentrated in a single formation layer. When the borehole is deviated, or when a bed exhibits relative dip with respect to the primary axis of the borehole, the signal and response may propagate through multiple layers and across multiple boundaries, resulting in a relative dip angle log that blends adjacent layers, and hence, a resistivity measurement that is a blending of the adjacent layers. In many cases, the effect of such a relative dip on the induction log is to make beds appear thicker, create separation of different sensor arrays and/or create gradual changes near the boundaries. It is known that thin beds are more affected by relative dip than thick beds, and resistive beds are more affected by relative dip than conductive beds.
Relative dip correction algorithms for array induction data have been implemented to remove the effects of relative dip in the response of the array induction logging tools. In such algorithms, removing the effect of the relative dip means that a log that is equivalent to what would be obtained if the well path was adjusted to be normal to the boundaries is achieved. However, current methods for automated relative dip corrections may result in results that are not always correct, especially in formations with interspersed thin beds or invasion, which may not be accurately accounted for by automated relative dip correction algorithms. A means for interpreting for or confirming the outputs from automated relative dip correction algorithms for array induction measurements allows for improved relative dip corrects and portrayals of invasion profiles.